Bipolar membranes have been prepared for a variety of uses including, for example, for improving desalination processes, as analogs to certain biological membranes, and for the production of acid and base from salt. The latter process has great potential usefulness since it allows the production of commercially important acids and bases from neutral salts, e.g., NaCl may be converted to HCl and NaOH. The potential for carrying out this type of conversion by means of bipolar membranes, the process known as electrodialytic water splitting, has not yet been realized as an industrial process primarily due to insufficient membrane life, stability and electrical properties of bipolar membranes which should have the following properties:
(1) sufficient mechanical strength PA1 (2) ability to operate at high current density PA1 (3) high permselectivity PA1 (4) low potential drop PA1 (5) stable properties
Additionally the bipolar membrane should be easily fabricated and have reproducible properties. Many preparations of bipolar membranes are reported in the literature. For example in U.S. Pat. No. 2,829,095, bipolar membranes are disclosed as being prepared by adhering two membranes consisting of oppositely charged ion exchange resins in an inert matrix bonded together either by fusion with heat and pressure or by use of a polyethyleneimine-epichlorohydrin paste. Anion and cation membranes have also been fused together as disclosed in U.S. Pat. No. 3,372,101 and British Pat. No. 1,038,777 by means of heat and pressure to give bipolar membranes. Additionally the application of an anionic polyelectrolyte paste to a cationic membrane which is then cured to yield a bipolar membrane has been described. Further, the preparation of bipolar membranes from a single sheet of material by selective functionalization of one side to cation or anion membrane followed by functionalization of the other side to the opposite type of membrane is disclosed for example in U.S. Pat. No. 3,654,125, and the formation of bipolar membranes in situ by passing a direct current through anion or cation membranes in contact with finely divided ion exchange resin slurries of charge opposite to that of the membrane is disclosed in published U.S. Patent Application No. 435,791, filed Jan. 23, 1974 (PB 230409) National Technical Information Service. In spite of such prior disclosures most membranes available up to the present time have been deficient in one or more of the properties desirable for bipolar membranes to be used for the process of electrodialytic water splitting on a practical level. It is thus seen that a need exists for a bipolar membrane of improved stability, high permselectivity and low potential drop.
In U.S. Pat. No. 4,116,889 (F. P. Chlanda, L. T. C. Lee and K. J. Liu) there is disclosed a method of preparing cast bipolar membranes by applying a second layer or coating of a cation exchange resin in a matrix polymer upon an anionic first layer preformed from a mixture of polyvinylidene fluoride, polyvinyl benzyl chloride solvent and multifunctional amine at least one of which is tertiary, mixed together in the proper proportions.
The subject matter of this application is also related to subject matter disclosed in U.S. Pat. No. 4,024,403 (G. J. Dege and K. J. Liu) entitled "Single Film, High Performance Bipolar Membrane" issued on May 17, 1977 and in U.S. Pat. No. 4,057,481 (L. T. C. Lee, G. J. Dege and K. J. Liu) issued on Nov. 8, 1977 entitled "High Performance, Quality Controlled Bipolar Membrane" and U.S. Pat. No. 4,083,768 (L. T. C. Lee and K. J. Liu) issued on Apr. 11, 1978, entitled "Preparation of High Performance Polyelectrolyte Membrane."